Plasma-assisted joining

ABSTRACT

Methods and apparatus for plasma-assisted joining of one or more parts together are provided. The joining process may include, for example, placing at least first and second joining areas in proximity to one another in a cavity, forming a plasma in the cavity by subjecting a gas to electromagnetic radiation in the presence of a plasma catalyst, and sustaining the plasma at least until the first and second joining areas are joined. Plasma catalysts, and methods and apparatus for igniting, modulating, and sustaining a joining plasma, are provided. Additional cavity shapes, and methods and apparatus for selective plasma-joining, are also provided.

CROSS-REFERENCE OF RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/430,414, filed May 7,2003 now U.S. Pat. No. 6,870,124 which is incorporated herein byreference.

Priority is claimed to U.S. Provisional Patent Application No.60/378,693, filed May 8, 2002, No. 60/430,677, filed Dec. 4, 2002, andNo. 60/435,278, filed Dec. 23, 2002, all of which are fully incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for plasma-assistedjoining two or more objects together and, particularly, to joining oneor more portions of such objects in various types of cavities using aplasma catalyst.

BACKGROUND

It is known that a plasma can be ignited by subjecting a gas to asufficient amount of electromagnetic radiation. It is also known thatradiation-induced plasmas may be used to join parts. Igniting andsustaining plasmas, however, can be slow, expensive, andenergy-consuming, especially when reduced pressures are needed to ignitethe plasma. Therefore, use of conventional plasma-assisted joining canlimit joining flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION

Methods and apparatus for plasma assisted joining of one or more partsare provided. In one embodiment consistent with this invention, a methodis provided for joining at least two parts using a plasma induced byelectromagnetic radiation having a wavelength of λ. The method caninclude placing at least first and second joining areas in proximity toone another in a cavity, forming a plasma in the cavity by subjecting agas to radiation in the presence of a plasma catalyst, and sustainingthe plasma at least until the first and second joining areas are joinedby directing the radiation into the cavity.

In another embodiment consistent with this invention, at least two partsto be joined (e.g., brazed) to each other may be placed in a cavityformed in a vessel. A plasma catalyst may be placed in or near thecavity. A brazing material (e.g., ring) may at least partially encircleone of the parts and abut the surface of the other part. In thisembodiment, the brazing results from at least partial melting thebrazing ring using a plasma induced by radiation.

A plasma catalyst for initiating, modulating, and sustaining a plasmaconsistent with this invention is also provided. The catalyst can bepassive or active. A passive plasma catalyst can include any objectcapable of inducing a plasma by deforming a local electric field(including an electromagnetic field) consistent with this invention,without necessarily adding additional energy. An active plasma catalyst,on the other hand, is any particle or high-energy wave packet capable oftransferring a sufficient amount of energy to a gaseous atom or moleculeto remove at least one electron from the gaseous atom or molecule in thepresence of electromagnetic radiation. In both cases, a plasma catalystcan improve, or relax, the environmental conditions required to ignite aplasma.

Additional plasma catalysts, and methods and apparatus for igniting,modulating, and sustaining a plasma consistent with this invention areprovided. Additional cavity shapes, and methods and apparatus forselectively plasma-joining in general, are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention will be apparent upon consideration ofthe following detailed description, taken in conjunction with theaccompanying drawings, in which like reference characters refer to likeparts throughout, and in which:

FIG. 1 shows a schematic diagram of an illustrative plasma systemconsistent with this invention;

FIG. 2 shows an illustrative embodiment of a portion of a plasma systemfor adding a powder plasma catalyst to a plasma cavity for igniting,modulating, or sustaining a plasma in a cavity consistent with thisinvention;

FIG. 3 shows an illustrative plasma catalyst fiber with at least onecomponent having a concentration gradient along its length consistentwith this invention;

FIG. 4 shows an illustrative plasma catalyst fiber with multiplecomponents at a ratio that varies along its length consistent with thisinvention;

FIG. 5 shows another illustrative plasma catalyst fiber that includes acore underlayer and a coating consistent with this invention;

FIG. 6 shows a cross-sectional view of the plasma catalyst fiber of FIG.5, taken from line 6-6 of FIG. 5, consistent with this invention;

FIG. 7 shows a cross-sectional view of an illustrative embodiment ofanother plasma-joining system, including an elongated plasma catalystthat extends through an ignition port consistent with this invention;

FIG. 8 shows an illustrative embodiment of an elongated plasma catalystthat can be used in the system of FIG. 7 consistent with this invention;

FIG. 9 shows another illustrative embodiment of an elongated plasmacatalyst that can be used in the system of FIG. 7 consistent with thisinvention;

FIG. 10 shows a side cross-sectional view of an illustrative embodimentof a portion of a plasma-joining system for directing ionizing radiationinto a plasma cavity consistent with this invention;

FIG. 11 shows a cross-sectional view of yet another illustrativeembodiment of a plasma-joining cavity and areas of objects to be joinedthat have been inserted into the cavity consistent with this invention;

FIG. 12 shows a cross-sectional view of still another illustrativeembodiment of a plasma-joining cavity, joining areas, and a brazing ringin the cavity consistent with this invention; and

FIG. 13 shows a cross-sectional view of yet another illustrativeembodiment of a plasma-joining cavity and joining areas where thejoining areas can act as a partial seal consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

This invention may relate to plasma-assisted joining of at least twoparts for a variety of applications, including, for example, brazing,welding, bonding, soldering, etc. In particular, various cavity shapes,plasma catalysts, and methods for joining are provided;

The following connonly owned U.S. Patent Applications are herebyincorporated by reference in their entireties: U.S. patent applicationSer. No. 10/513,221; U.S. patent application Ser. No. 10/513,393; PCTApplication No. US03/14132, now expired; U.S. patent application Ser.No. 10/513,394; U.S. patent application Ser. No. 10/513,305; U.S. patentapplication Ser. No. 10/513,607; PCT Application No. US03/14034, nowexpired; U.S. patent application Ser. No. 10/430,416; U.S. patentapplication Ser. No. 10/430,415; PCT Application No. US03/14133, nowexpired; U.S. patent application Ser. No. 10/513,605; U.S. patentapplication Ser. No. 10/513,309; U.S. patent application Ser. No.10/513,220; U.S. patent application Ser. No. 10/513,397; U.S. patent:application Ser. No. 10/513,605; PCT Application No. US03/14137, nowexpired; U.S. patent application Ser. No. 10/430,426, now U.S. Pat. No.7,132,621; PCT Application No. US03/14121, now expired; U.S. patentapplication Ser. No. 10/513,604; and PCT Application No. US03/14135, nowexpired.

Illustrative Plasma-Assisted Joining System

FIG. 1 shows illustrative plasma-assisted joining system 10 consistentwith one aspect of this invention. In this embodiment, plasma-joiningcavity 12 is formed in vessel 13 that is positioned inside radiationchamber (i.e., applicator) 14. In another embodiment (not shown), vessel13 and radiation chamber 14 are the same, thereby eliminating the needfor two separate components.

Vessel 13 can include one or more radiation-transmissive insulatinglayers to improve its thermal insulation properties withoutsignificantly shielding cavity 12 from the radiation used to form aplasma. It will be appreciated that more than one cavity can be formedin vessel 13. In one embodiment, multiple cavities can be formed invessel 13 and those cavities can be in fluid communication with oneanother. At least one portion of each of parts 11 and 16 can be placedin cavity 12. As described more fully below, plasma-assisted joiningconsistent with this invention can involve generating a plasma inselected regions and preventing a plasma in others.

As used herein, a plasma-joining cavity is any localized volume capableof igniting, modulating, and/or sustaining a plasma. Thus, it will beappreciated that a cavity consistent with this invention need not becompletely closed, and may indeed be open. It is known that a plasma canbe ignited by subjecting a gas to a sufficient amount of radiation. Theplasma may then be modulated or sustained by direct absorption of theradiation, but may be assisted by a plasma catalyst during joining.

In one embodiment, cavity 12 is formed in a vessel made of ceramic. Dueto the extremely high temperatures that can be achieved with plasmasconsistent with this invention, a ceramic capable of operating at, forexample, about 3,000 degrees Fahrenheit can be used. The ceramicmaterial can include, by weight, 29.8% silica, 68.2% alumina, 0.4%ferric oxide, 1% titania, 0.1% lime, 0.1% magnesia, 0.4% alkalies, whichis sold under Model No. LW-30 by New Castle Refractories Company, of NewCastle, Pa. It will be appreciated by those of ordinary skill in theart, however, that other materials, such as quartz, and those differentfrom the one described above, can also be used consistent with theinvention. It will also be appreciated that because the operatingtemperature can be different for different joining processes, thematerial used to make the vessel may only need to withstand temperaturessubstantially below 3,000 degree Fahrenheit, such as 2,500 degrees orabout 1,000 degree Fahrenheit, or even lower.

In one successful experiment, a catalyzed joining plasma was formed in apartially open cavity inside a first brick and topped with a secondbrick. The cavity had dimensions of about 2 inches by about 2 inches byabout 1.5 inches. At least two holes were also provided in the brick incommunication with the cavity: one for viewing the plasma and at leastone hole for providing the gas. The size of the cavity can depend on thedesired joining process being performed. Also, the cavity can beconfigured to prevent the plasma from rising/floating away from theprimary processing region.

As shown in FIG. 1, for example, cavity 12 can be connected to one ormore gas sources 24 (e.g., a source of argon, nitrogen, hydrogen, xenon,krypton) by line 20 and control valve 22, which may be powered by powersupply 28. Line 20 may be tubing or any other device capable ofdelivering a gas. In one embodiment, the diameter of the tube issufficiently small to prevent radiation leakage (e.g., between about1/16 inch and about ¼ inch, such as about ⅛″). Also, if desired, avacuum pump (not shown) can be connected to the chamber to remove anyundesirable fumes that may be generated during joining. Although notshown in FIG. 1, cavity 12 and chamber 14 can have a separate gas portfor removing gas.

A radiation leak detector (not shown) was installed near source 26 andwaveguide 30 and connected to a safety interlock system to automaticallyturn off the radiation (e.g., microwave) power supply if a leak above apredefined safety limit, such as one specified by the FCC and/or OSHA(e.g., 5 mW/cm²), was detected.

Radiation source 26, which may be powered by electrical power supply 28,can direct radiation energy into chamber 14 through one or morewaveguides 30. It will be appreciated by those of ordinary skill in theart that source 26 can be connected directly to cavity 12 or chamber 14,thereby eliminating waveguide 30. The radiation energy entering cavity12 can be used to ignite a plasma within the cavity with the assistanceof a plasma catalyst. This plasma can be substantially sustained andconfined to the cavity by coupling additional radiation with thecatalyst.

Radiation energy can be supplied through optional circulator 32 andtuner 34 (e.g., 3-stub tuner). Tuner 34 can be used to minimize thereflected power as a function of changing ignition or processingconditions, especially before the plasma has formed because radiationpower, for example, will be strongly absorbed by the plasma.

As explained more fully below, the location of cavity 12 in chamber 14may not be critical if chamber 14 supports multiple modes, andespecially when the modes are continually or periodically mixed. As alsoexplained more fully below, motor 36 can be connected to mode-mixer 38for making the time-averaged radiation energy distribution substantiallyuniform throughout chamber 14. Furthermore, as shown in FIG. 1, forexample, window 40 (e.g., a quartz window) can be disposed in one wallof chamber 14 adjacent to cavity 12, permitting temperature sensor 42(e.g., an optical pyrometer) to be used to view a process inside cavity12. In one embodiment, the optical pyrometer output can increase fromzero volts as the temperature rises to within the tracking range.

Sensor 42 can develop output signals as a function of the temperature orany other monitorable condition associated with work piece 11 withincavity 12 and provide the signals to controller 44. Dual temperaturesensing and heating, as well as automated cooling rate and gas flowcontrols, can also be used. Controller 44 in turn can be used to controloperation of power supply 28, which can have one output connected tosource 26 as described above and another output connected to valve 22 tocontrol gas flow into cavity 12. Although not shown in FIG. 1, chamber14 can have a separate gas port for removing exhaust fumes.

The invention has been practiced with equal success employing microwavesources at both 915 MHz and 2.45 GHz provided by Communications andPower Industries (CPI), although radiation having any frequency lessthan about 333 GHz can be used. The 2.45 GHz system providedcontinuously variable microwave power from about 5.0 kilowatts to about5.0 kilowatts. A 3-stub tuner allowed impedance matching for maximumpower transfer and a dual directional coupler (not shown) was used tomeasure forward and reflected powers. Also, optical pyrometers were usedfor remote sensing of the sample temperature.

As mentioned above, radiation having any frequency less than about 333GHz can be used consistent with this invention. For example,frequencies, such as power line frequencies (about 50 Hz to about 60Hz), can be used, although the pressure of the gas from which thejoining plasma is formed may be lowered to assist with plasma ignition.Also, any radio frequency or microwave frequency can be used consistentwith this invention, including frequencies greater than about 100 kHz.In most cases, the gas pressure for such relatively high frequenciesneed not be lowered to ignite, modulate, or sustain a plasma, therebyenabling many plasma-processes to occur at atmospheric pressures andabove.

The equipment was computer controlled using LabView 6i software, whichprovided real-time temperature monitoring and microwave power control.Noise was reduced by using sliding averages of suitable number of datapoints. Also, to improve speed and computational efficiency, the numberof stored data points in the buffer array was limited by using shiftregisters and buffer sizing. The pyrometer measured the temperature of asensitive area of about 1 cm², which was used to calculate an averagetemperature. The pyrometer sensed radiant intensities at two wavelengthsand fit those intensities using Planck's law to determine thetemperature.

It will be appreciated, however, that other devices and methods formonitoring and controlling temperature are also available and can beused consistent with this invention. Control software that can be usedconsistent with this invention is described, for example, in commonlyowned, concurrently filed PCT Application No. US03/14135, now expired,which is hereby incorporated by reference in its entirety.

Chamber 14 had several glass-covered viewing ports with radiationshields and one quartz window for pyrometer access. Several ports forconnection to a vacuum pump and a gas source were also provided,although not necessarily used.

System 10 also included a closed-loop deionized water-cooling system(not shown) with an external heat exchanger cooled by tap water. Duringoperation, the deionized water first cooled the magnetron, then theload-dump in the circulator (used to protect the magnetron), and finallythe radiation chamber through water channels welded on the outer surfaceof the chamber.

Plasma Catalysts

A plasma catalyst consistent with this invention can include one or moredifferent materials and may be either passive or active. A plasmacatalyst can be used, among other things, to ignite, modulate, and/orsustain a plasma at a gas pressure that is less than, equal to, orgreater than atmospheric pressure.

One method of forming a joining plasma consistent with this inventioncan include subjecting a gas in a cavity to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of a passiveplasma catalyst. A passive plasma catalyst consistent with thisinvention can include any object capable of inducing a plasma bydeforming a local electric field (e.g., an electromagnetic field)consistent with this invention, without necessarily adding additionalenergy through the catalyst, such as by applying an electric voltage tocreate a spark.

A passive plasma catalyst consistent with this invention can be, forexample, a nano-particle or a nano-tube. As used herein, the term“nano-particle” can include any particle having a maximum physicaldimension less than about 100 nm that is at least electricallysemi-conductive. Also, both single-walled and multi-walled carbonnano-tubes, doped and undoped, can be particularly effective forigniting plasmas consistent with this invention because of theirexceptional electrical conductivity and elongated shape. The nano-tubescan have any convenient length and can be a powder fixed to a substrate.If fixed, the nano-tubes can be oriented randomly on the surface of thesubstrate or fixed to the substrate (e.g., at some predeterminedorientation) while the plasma is ignited or sustained.

A passive plasma catalyst can also be a powder consistent with thisinvention, and need not comprise nano-particles or nano-tubes. It can beformed, for example, from fibers, dust particles, flakes, sheets, etc.When in powder form, the catalyst can be suspended, at leasttemporarily, in a gas. By suspending the powder in the gas, the powdercan be quickly dispersed throughout the cavity and more easily consumed,if desired.

In one embodiment, the powder catalyst can be carried into the cavityand at least temporarily suspended with a carrier gas. The carrier gascan be the same or different from the gas that forms the plasma. Also,the powder can be added to the gas prior to being introduced to thecavity. For example, as shown in FIG. 2, radiation source 52 can supplyradiation to radiation chamber 55, in which plasma cavity 60 is placed.Powder source 65 can provide catalytic powder 70 into gas stream 75. Inan alternative embodiment, powder 70 can be first added to cavity 60 inbulk (e.g., in a pile) and then distributed in cavity 60 in any numberof ways, including flowing a gas through or over the bulk powder. Inaddition, the powder can be added to the gas for igniting, modulating,or sustaining a plasma by moving, conveying, drizzling, sprinkling,blowing, or otherwise, feeding the powder into or within the cavity.

In one experiment, a joining plasma was ignited in a cavity by placing apile of carbon fiber powder in a copper pipe that extended into thecavity. Although sufficient radiation was directed into the cavity, thecopper pipe shielded the powder from the radiation and no plasmaignition took place. However, once a carrier gas began flowing throughthe pipe, forcing the powder out of the pipe and into the cavity, andthereby subjecting the powder to the radiation, a plasma was nearlyinstantaneously ignited in the cavity. Such instantaneous ignition cansubstantially eliminate potentially damaging radiation that couldotherwise reflect back into the radiation source.

A powder plasma catalyst consistent with this invention can besubstantially non-combustible, thus it need not contain oxygen or burnin the presence of oxygen. Thus, as mentioned above, the catalyst caninclude a metal, carbon, a carbon-based alloy, a carbon-based composite,an electrically conductive polymer, a conductive silicone elastomer, apolymer nano-composite, an organic-inorganic composite, and anycombination thereof.

Also, powder catalysts can be substantially uniformly distributed in theplasma cavity (e.g., when suspended in a gas), and plasma ignition canbe precisely controlled within the cavity consistent with thisinvention. Uniform ignition can be important in certain applications,including those applications requiring brief plasma exposures, such asin the form of one or more bursts. Still, a certain amount of time canbe required for a powder catalyst to distribute itself throughout acavity, especially in complicated, multi-chamber cavities. Therefore,consistent with another aspect of this invention, a powder catalyst canbe introduced into the cavity through a plurality of ignition ports tomore rapidly obtain a more uniform catalyst distribution therein (seebelow).

In addition to powder, a passive plasma catalyst consistent with thisinvention can include, for example, one or more microscopic ormacroscopic fibers, sheets, needles, threads, strands, filaments, yarns,twines, shavings, slivers, chips, woven fabrics, tape, whiskers, or anycombination thereof. In these cases, the plasma catalyst can have atleast one portion with one physical dimension substantially larger thananother physical dimension. For example, the ratio between at least twoorthogonal dimensions should be at least about 1:2, but could be greaterthan about 1:5, or even greater than about 1:10.

Thus, a passive plasma catalyst can include at least one portion ofmaterial that is relatively thin compared to its length. A bundle ofcatalysts (e.g., fibers) may also be used and can include, for example,a section of graphite tape. In one experiment, a section of tape havingapproximately thirty thousand strands of graphite fiber, each about 2-3microns in diameter, was successfully used. The number of fibers in andthe length of a bundle are not critical to igniting, modulating, orsustaining the plasma. For example, satisfactory results have beenobtained using a section of graphite tape about one-quarter inch long.One type of carbon fiber that has been successfully used consistent withthis invention is sold under the trademark Magnamite®, Model No.AS4C-GP3K, by the Hexcel Corporation, of Anderson, S.C. Also,silicon-carbide fibers have been successfully used.

A passive plasma catalyst consistent with another aspect of thisinvention can include one or more portions that are, for example,substantially spherical, annular, pyramidal, cubic, planar, cylindrical,rectangular or elongated.

The passive plasma catalysts discussed above include at least onematerial that is at least electrically semi-conductive. In oneembodiment, the material can be highly electrically conductive. Forexample, a passive plasma catalyst consistent with this invention caninclude a metal, an inorganic material, carbon, a carbon-based alloy, acarbon-based composite, an electrically conductive polymer, a conductivesilicone elastomer, a polymer nano-composite, an organic-inorganiccomposite, or any combination thereof. Some of the possible inorganicmaterials that can be included in the plasma catalyst include carbon,silicon carbide, molybdenum, platinum, tantalum, tungsten, carbonnitride, and aluminum, although other electrically conductive inorganicmaterials are believed to work just as well.

In addition to one or more electrically conductive materials, a passiveplasma catalyst consistent with this invention can include one or moreadditives (which need not be electrically conductive). As used herein,the additive can include any material that a user wishes to add to theplasma. For example, in doping semiconductors and other materials, oneor more dopants can be added to the plasma through the catalyst. See,e.g., commonly owned, concurrently filed U.S. patent application Ser.No. 10/513,397, which is hereby incorporated by reference in itsentirety. The catalyst can include the dopant itself, or it can includea. precursor material that, upon decomposition, can form the dopant.Thus, the plasma catalyst can include one or more additives and one ormore electrically conductive materials in any desirable ratio, dependingon the ultimate desired composition of the plasma and the process usingthe plasma.

The ratio of the electrically conductive components to the additives ina passive plasma catalyst can vary over time while being consumed. Forexample, during ignition, the plasma catalyst could desirably include arelatively large percentage of electrically conductive components toimprove the ignition conditions. On the other hand, if used whilemodulating or sustaining the plasma, the catalyst could include arelatively large percentage of additives that may be desirable in ajoining process. It will be appreciated by those of ordinary skill inthe art that the component ratio of the plasma catalyst used to igniteand, later, to sustain the plasma, could be the same.

A predetermined ratio profile can be used to simplify many plasmaprocesses. In many conventional plasma processes, the components withinthe plasma are added as necessary, but such addition normally requiresprogrammable equipment to add the components according to apredetermined schedule. However, consistent with this invention, theratio of components in the catalyst can be varied, and thus the ratio ofcomponents in the plasma itself can be automatically varied. That is,the ratio of components in the plasma at any particular time can dependon which of the catalyst portions is currently being consumed by theplasma. Thus, the catalyst component ratio can be different at differentlocations within the catalyst. And, the current ratio of components in aplasma can depend on the portions of the catalyst currently and/orpreviously consumed, especially when the flow rate of a gas passingthrough the plasma chamber is relatively slow.

A passive plasma catalyst consistent with this invention can behomogeneous, inhomogeneous, or graded. Also, the plasma catalystcomponent ratio can vary continuously or discontinuously throughout thecatalyst. For example, in FIG. 3, the ratio can vary smoothly forming agradient along a length of catalyst 100. Catalyst 100 can include astrand of material that includes a relatively low concentration of acomponent at section 105 and a continuously increasing concentrationtoward section 110.

Alternatively, as shown in FIG. 4, the ratio can vary discontinuously ineach portion of catalyst 120, which includes, for example, alternatingsections 125 and 130 having different concentrations. It will beappreciated that catalyst 120 can have more than two section types.Thus, the catalytic component ratio being consumed by the plasma canvary in any predetermined fashion. In one embodiment, when the plasma ismonitored and a particular additive is detected, further processing canbe automatically commenced or terminated.

Another way to vary the ratio of components in a sustained plasma is byintroducing multiple catalysts having different component ratios atdifferent times or different rates. For example, multiple catalysts canbe introduced at approximately the same location or at differentlocations within the cavity. When introduced at different locations, theplasma formed in the cavity can have a component concentration gradientdetermined by the locations of the various catalysts. Thus, an automatedsystem can include a device by which a consumable plasma catalyst ismechanically inserted before and/or during plasma igniting, modulating,and/or sustaining.

A passive plasma catalyst consistent with this invention can also becoated. In one embodiment, a catalyst can include a substantiallynon-electrically conductive coating deposited on the surface of asubstantially electrically conductive material. Alternatively, thecatalyst can include a substantially electrically conductive coatingdeposited on the surface of a substantially electrically non-conductivematerial. FIGS. 5 and 6, for example, show fiber 140, which includesunderlayer 145 and coating 150. In one embodiment, a plasma catalystincluding a carbon core is coated with nickel to prevent oxidation ofthe carbon.

A single plasma catalyst can also include multiple coatings. If thecoatings are consumed during contact with the plasma, the coatings couldbe introduced into the plasma sequentially, from the outer coating tothe innermost coating, thereby creating a time-release mechanism. Thus,a coated plasma catalyst can include any number of materials, as long asa portion of the catalyst is at least electrically semi-conductive.

Consistent with another embodiment of this invention, a plasma catalystcan be located entirely within a radiation cavity to substantiallyreduce or prevent radiation energy leakage. In this way, the plasmacatalyst does not electrically or magnetically couple with the vesselcontaining the cavity or to any electrically conductive object outsidethe cavity. This prevents sparking at the ignition port and can preventradiation from leaking outside the cavity during the ignition andpossibly later if the plasma is sustained. In one embodiment, thecatalyst can be located at a tip of a substantially electricallynon-conductive extender that extends through an ignition port.

FIG. 7, for example, shows radiation chamber 160 in which plasma-joiningcavity 165 is placed. Plasma catalyst 170 is elongated and extendsthrough ignition port 175. As shown in FIG. 8, and consistent with thisinvention, catalyst 170 can include electrically conductive distalportion 180 (which is placed in chamber 160) and electricallynon-conductive portion 185 (which is placed substantially outsidechamber 160, but can extend somewhat into chamber 160). Thisconfiguration can prevent an electrical connection (e.g., sparking)between distal portion 180 and chamber 160.

In another embodiment, shown in FIG. 9, the catalyst can be formed froma plurality of electrically conductive segments 190 separated by andmechanically connected to a plurality of electrically non-conductivesegments 195. In this embodiment, the catalyst can extend through theignition port between a point inside the cavity and another pointoutside the cavity, but the electrically discontinuous profilesignificantly can prevent sparking and energy leakage.

Another method of forming a plasma consistent with this inventionincludes subjecting a gas in a cavity to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of an activeplasma catalyst, which generates or includes at least one ionizingparticle.

An active plasma catalyst consistent with this invention can be anyparticle or high-energy wave packet capable of transferring a sufficientamount of energy to a gaseous atom or molecule to remove at least oneelectron from the gaseous atom or molecule in the presence ofelectromagnetic radiation. Depending on the source, the ionizingparticles can be directed into the cavity in the form of a focused orcollimated beam, or they may be sprayed, spewed, sputtered, or otherwiseintroduced.

For example, FIG.10 shows radiation source 200 directing radiation intoplasma cavity 210, which can be positioned inside of chamber 205. Plasmacavity 210 may permit a gas to flow through it via ports 215 and 216 ifdesired. Source 220 directs ionizing particles 225 into cavity 210.Source 220 can be protected, for example, by a metallic screen, whichallows the ionizing particles to pass through but shields source 220from radiation. If necessary, source 220 can be water-cooled.

Examples of ionizing particles consistent with this invention caninclude x-ray particles, gamma ray particles, alpha particles, betaparticles, neutrons, protons, and any combination thereof. Thus, anionizing particle catalyst can be charged (e.g., an ion from an ionsource) or uncharged and can be the product of a radioactive fissionprocess. In one embodiment, the vessel in which the plasma cavity isformed could be entirely or partially transmissive to the ionizingparticle catalyst. Thus, when a radioactive fission source is locatedoutside the cavity, the source can direct the fission products throughthe vessel to ignite the plasma. The radioactive fission source can belocated inside the radiation chamber to substantially prevent thefission products (i.e., the ionizing particle catalyst) from creating asafety hazard.

In another embodiment, the ionizing particle can be a free electron, butit need not be emitted in a radioactive decay process. For example, theelectron can be introduced into the cavity by energizing the electronsource (such as a metal), such that the electrons have sufficient energyto escape from the source. The electron source can be located inside thecavity, adjacent the cavity, or even in the cavity wall. It will beappreciated by those of ordinary skill in the art that the anycombination of electron sources is possible. A common way to produceelectrons is to heat a metal, and these electrons can be furtheraccelerated by applying an electric field.

In addition to electrons, free energetic protons can also be used tocatalyze a plasma. In one embodiment, a free proton can be generated byionizing hydrogen and, optionally, accelerated with an electric field.

Multi-mode Radiation Cavities

A radiation waveguide, cavity, or chamber can be designed to support orfacilitate propagation of at least one electromagnetic radiation mode.As used herein, the term “mode” refers to a particular pattern of anystanding or propagating electromagnetic wave that satisfies Maxwell'sequations and the applicable boundary conditions (e.g., of the cavity).In a waveguide or cavity, the mode can be any one of the variouspossible patterns of propagating or standing electromagnetic fields.Each mode is characterized by its frequency and polarization of theelectric field and/or the magnetic field vectors. The electromagneticfield pattern of a mode depends on the frequency, refractive indices ordielectric constants, and waveguide or cavity geometry.

A transverse electric (TE) mode is one whose electric field vector isnormal to the direction of propagation. Similarly, a transverse magnetic(TM) mode is one whose magnetic field vector is normal to the directionof propagation. A transverse electric and magnetic (TEM) mode is onewhose electric and magnetic field vectors are both normal to thedirection of propagation. A hollow metallic waveguide does not typicallysupport a normal TEM mode of radiation propagation. Even thoughradiation appears to travel along the length of a waveguide, it may doso only by reflecting off the inner walls of the waveguide at someangle. Hence, depending upon the propagation mode, the radiation (e.g.,microwave) may have either some electric field component or somemagnetic field component along the axis of the waveguide (often referredto as the z-axis).

The actual field distribution inside a cavity or waveguide is asuperposition of the modes therein. Each of the modes can be identifiedwith one or more subscripts (e.g., TE₁₀ (“tee ee one zero”). Thesubscripts normally specify how many “half waves” at the guidewavelength are contained in the x and y directions. It will beappreciated by those skilled in the art that the guide wavelength can bedifferent from the free space wavelength because radiation propagatesinside the waveguide by reflecting at some angle from the inner walls ofthe waveguide. In some cases, a third subscript can be added to definethe number of half waves in the standing wave pattern along the z-axis.

For a given radiation frequency, the size of the waveguide can beselected to be small enough so that it can support a single propagationmode. In such a case, the system is called a single-mode system (i.e., asingle-mode applicator). The TE₁₀ mode is usually dominant in arectangular single-mode waveguide.

As the size of the waveguide (or the cavity to which the waveguide isconnected) increases, the waveguide or applicator can sometimes supportadditional higher order modes forming a multi-mode system. When manymodes are capable of being supported simultaneously, the system is oftenreferred to as highly moded.

A simple, single-mode system has a field distribution that includes atleast one maximum and/or minimum. The magnitude of a maximum largelydepends on the amount of radiation supplied to the system. Thus, thefield distribution of a single mode system is strongly varying andsubstantially non-uniform.

Unlike a single-mode cavity, a multi-mode cavity can support severalpropagation modes simultaneously, which, when superimposed, results in acomplex field distribution pattern. In such a pattern, the fields tendto spatially smear and, thus, the field distribution usually does notshow the same types of strong minima and maxima field values within thecavity. In addition, as explained more fully below, a mode-mixer can beused to “stir” or “redistribute” modes (e.g., by mechanical movement ofa radiation reflector). This redistribution desirably provides a moreuniform time-averaged field distribution within the cavity.

A multi-mode cavity consistent with this invention can support at leasttwo modes, and may support many more than two modes. Each mode has amaximum electric field vector. Although there may be two or more modes,one mode may be dominant and has a maximum electric field vectormagnitude that is larger than the other modes. As used herein, amulti-mode cavity may be any cavity in which the ratio between the firstand second mode magnitudes is less than about 1:10, or less than about1:5, or even less than about 1:2. It will be appreciated by those ofordinary skill in the art that the smaller the ratio, the moredistributed the electric field energy between the modes, and hence themore distributed the radiation energy is in the cavity.

The distribution of plasma within a processing cavity may stronglydepend on the distribution of the applied radiation. For example, in apure single mode system, there may only be a single location at whichthe electric field is a maximum. Therefore, a strong plasma may onlyform at that single location. In many applications, such a stronglylocalized plasma could undesirably lead to non-uniform plasma treatmentor heating (i.e., localized overheating and underheating).

Whether or not a single or multi-mode cavity is used consistent withthis invention, it will be appreciated by those of ordinary skill in theart that the cavity in which the plasma is formed can be completelyclosed or partially open. For example, in certain applications, such asin plasma-assisted furnaces, the cavity could be entirely closed. See,for example, commonly owned, concurrently filed PCT Application No.US03/14133, now expired, which is fully incorporated herein byreference. In other applications, however, it may be desirable to flow agas through the cavity, and therefore the cavity must be open to somedegree. In this way, the flow, type, and pressure of the flowing gas canbe varied over time. This may be desirable because certain gases withlower ionization potentials, such as argon, are easier to ignite but mayhave other undesirable properties during subsequent plasma processing.

Mode-mixing

For many joining applications, a cavity containing a uniform plasma isdesirable. However, because radiation can have a relatively longwavelength (e.g., several tens of centimeters), obtaining asubstantially uniform plasma distribution can be difficult to achieve.As a result, consistent with one aspect of this invention, the radiationmodes in a multi-mode joining cavity can be mixed, or redistributed,over a period of time. Because the field distribution within the cavitymust satisfy all of the boundary conditions set by the inner surface ofthe cavity, those field distributions can be changed by changing theposition of any portion of that inner surface.

In one embodiment consistent with this invention, a movable reflectivesurface can be located inside the radiation cavity. The shape and motionof the reflective surface should, when combined, change the innersurface of the cavity during motion. For example, an “L” shaped metallicobject (i.e., “mode-mixer”) when rotated about any axis will change thelocation or the orientation of the reflective surfaces in the cavity andtherefore change the radiation distribution therein. Any otherasymmetrically shaped object can also be used (when rotated), butsymmetrically shaped objects can also work, as long as the relativemotion (e.g., rotation, translation, or a combination of both) causessome change in location or orientation of the reflective surfaces. Inone embodiment, a mode-mixer can be a cylinder that is rotable about anaxis that is not the cylinder's longitudinal axis.

Each mode of a multi-mode cavity may have at least one maximum electricfield vector, but each of these vectors could occur periodically acrossthe inner dimension of the cavity. Normally, these maxima are fixed,assuming that the frequency of the radiation does not change. However,by moving a mode-mixer such that it interacts with the radiation, it ispossible to move the positions of the maxima. For example, mode-mixer 38of FIG. 1 can be used to optimize the field distribution within cavity12 such that the plasma ignition conditions and/or the plasma sustainingconditions are optimized. Thus, once a plasma is excited, the positionof the mode-mixer can be changed to move the position of the maxima fora uniform time-averaged plasma process (e.g., heating).

Thus, consistent with this invention, mode-mixing can be useful duringplasma ignition. For example, when an electrically conductive fiber isused as a plasma catalyst, it is known that the fiber's orientation canstrongly affect the minimum plasma-ignition conditions. It has beenreported, for example, that when such a fiber is oriented at an anglethat is greater than 60° to the electric field, the catalyst does littleto improve, or relax, these conditions. By moving a reflective surfaceeither in or near the cavity, however, the electric field distributioncan be significantly changed.

Mode-mixing can also be achieved by launching the radiation into theapplicator chamber through, for example, a rotating waveguide joint thatcan be mounted inside the applicator chamber. The rotary joint can bemechanically moved (e.g., rotated) to effectively launch the radiationin different directions in the radiation chamber. As a result, achanging field pattern can be generated inside the applicator chamber.

Mode-mixing can also be achieved by launching radiation in the radiationchamber through a flexible waveguide. In one embodiment, the waveguidecan be mounted inside the chamber. In another embodiment, the waveguidecan extend into the chamber. The position of the end portion of theflexible waveguide can be continually or periodically moved (e.g., bent)in any suitable manner to launch the radiation (e.g., microwaveradiation) into the chamber at different directions and/or locations.This movement can also result in mode-mixing and facilitate more uniformplasma processing (e.g., heating) on a time-averaged basis.Alternatively, this movement can be used to optimize the location of aplasma for ignition or other plasma-assisted process.

If the flexible waveguide is rectangular, a simple twisting of the openend of the waveguide will rotate the orientation of the electric and themagnetic field vectors in the radiation inside the applicator chamber.Then, a periodic twisting of the waveguide can result in mode-mixing aswell as rotating the electric field, which can be used to assistignition, modulation, or sustaining of a plasma.

Thus, even if the initial orientation of the catalyst is perpendicularto the electric field, the redirection of the electric field vectors canchange the ineffective orientation to a more effective one. Thoseskilled in the art will appreciate that mode-mixing can be continuous,periodic, or preprogrammed.

In addition to plasma ignition, mode-mixing can be useful duringsubsequent plasma processing to reduce or create (e.g., tune) “hotspots” in the chamber. When a processing cavity only supports a smallnumber of modes (e.g., less than 5), one or more localized electricfield maxima can lead to “hot spots” (e.g., within cavity 12). In oneembodiment, these hot spots could be configured to coincide with one ormore separate, but simultaneous, plasma ignitions or processing events.Thus, the plasma catalyst can be located at one or more of thoseignition or subsequent processing positions.

Multi-location Ignition

A plasma can be ignited using multiple plasma catalysts at differentlocations. In one embodiment, multiple fibers can be used to ignite theplasma at different points within the cavity. Such multi-point ignitioncan be especially beneficial when a uniform plasma ignition is desired.For example, when a plasma is modulated at a high frequency (i.e., tensof Hertz and higher), or ignited in a relatively large volume, or both,substantially uniform instantaneous striking and restriking of theplasma can be improved. Alternatively, when plasma catalysts are used atmultiple points, they can be used to sequentially ignite a plasma atdifferent locations within a plasma chamber by selectively introducingthe catalyst at those different locations. In this way, a plasmaignition gradient can be controllably formed within the cavity, ifdesired.

Also, in a multi-mode joining cavity, random distribution of thecatalyst throughout multiple locations in the cavity increases thelikelihood that at least one of the fibers, or any other passive plasmacatalyst consistent with this invention, is optimally oriented with theelectric field lines. Still, even where the catalyst is not optimallyoriented (not substantially aligned with the electric field lines), theignition conditions are improved.

Furthermore, because a catalytic powder can be suspended in a gas, it isbelieved that each powder particle may have the effect of being placedat a different physical location within the cavity, thereby improvingignition uniformity within the cavity.

Dual-Cavity Plasma Igniting/Sustaining

A dual-cavity arrangement can be used to ignite and sustain a joiningplasma consistent with this invention. In one embodiment, a systemincludes at least an ignition cavity and a second cavity in fluidcommunication with the ignition cavity. To ignite a plasma, a gas in theignition cavity can be subjected to electromagnetic radiation having afrequency less than about 333 GHz, optionally in the presence of aplasma catalyst. In this way, the proximity of the first and secondcavities may permit a plasma formed in the first cavity to ignite aplasma in the second cavity, which may be sustained with additionalelectromagnetic radiation.

In one embodiment of this invention, the ignition cavity can be verysmall and designed primarily, or solely, for plasma ignition. In thisway, very little radiation energy may be required to ignite the plasma,permitting easier ignition, especially when a plasma catalyst is usedconsistent with this invention.

In one embodiment, the ignition cavity may be a substantially singlemode cavity and the second cavity a multi-mode cavity. When the ignitioncavity only supports a single mode, the electric field distribution maystrongly vary within the cavity, forming one or more precisely locatedelectric field maxima. Such maxima are normally the first locations atwhich plasmas ignite, making them ideal points for placing plasmacatalysts. It will be appreciated, however, that when a plasma catalystis used, it need not be placed in the electric field maximum and, manycases, need not be oriented in any particular direction.

Illustrative Plasma-assisted Joining

As used herein, the term “plasma-assisted joining,” or simply “joining,”refers to any operation, or combination of operations, thatinvolves theuse of a plasma to put or bring together at least two parts and joiningthem so as to form a unit. Thus, plasma-assisted joining can include,for example, brazing, welding, bonding, soldering, fusing, etc.

Plasma-assisted joining consistent with this invention may includeplacing at least a first joining area of a first part and a secondjoining area of a second part in proximity to one another in or near ajoining cavity. The joining areas of the parts can be, for example,metal, non-metal, or a combination thereof. As described above, a gasmay be directed into the cavity in an amount sufficient to at least forma plasma and then to either modulate or sustain the plasma. The plasmacan be modulated or sustained in the cavity by continued absorption ofradiation until the at least two joining areas are joined to one anotheror until a predetermined temperature indicative of a particular joiningstatus is attained.

As described more fully above, the distribution of plasma within ajoining cavity can depend on the distribution of the applied radiation.For example, a strong plasma may only form at locations at which theelectric field (e.g., the radiation density) is large. Because thestrength of the electric field can be reduced, or even made to be zeronear the inner surface of the cavity, plasma formation can be preventedat that surface. Therefore, the inner surface of a cavity can becontoured to selectively form a plasma only where parts should be heatedto allow proper joining. Similarly, the electric field may not besufficiently strong to form plasma at other locations, such as where theinner surface of the cavity (e.g., a metallic surface) is separated fromthe surface of the part by a distance less than about λ/4. In addition,controlling the plasma volume in the proximity of a particular surfacearea of the object can be used to control the thermal energy deliveredto that surface area.

FIG. 11, for example, shows a cross-sectional view of plasma processingcavity 360 formed in vessel 313 consistent with this invention. Joiningareas 305 and 315 of parts 300 and 310, respectively, can be located incavity 360. In this example, vessel 313 can be placed inside radiationchamber 303, which can be electrically conductive, and vessel 313, whichcan be substantially transparent to plasma-forming radiation. In thisexample, inner wall 365 of cavity 360 may be custom-configured toconform, in at least some areas, to the parts, especially where a smallgap is desirable to suppress plasma heating. Alternatively, plasmaformation can be prevented by shielding a portion of the object'ssurface with a substantially radiation opaque material, yet notnecessarily by a contoured cavity surface.

Inner surface 365 can be shaped to hold parts 300 and 310 such thatjoining areas 305 and 315 are spaced from inner surface 365 by distance325 of at least about λ/4, wherein λ is the wavelength of the appliedradiation. Other surface portions of parts 300 and 310 can be spacedfrom inner surface 365 by a distance of less than about λ/4, such as bydistance 326 or 327. Also, distance 335, located between surface 330 andsurface 340 of cavity 360 at aperture 320, can be less than about λ/4 tosubstantially confine the plasma within cavity 360 and substantiallyprevent any plasma from forming in the region where the gaps are lessthan about λ/4 there.

An opening in the cavity wall 313 can be present and sufficient,however, to permit a gas to flow therethrough, if desired. Also, it willbe appreciated that although one gas port is shown, aperture 320 can actas another port, or vessel 313 can have more than one gas port. It willalso be appreciated that object 300 can act as a partial seal ataperture 320 and can be supported there.

An electric bias may be applied to any of the joining areas during aplasma-assisted joining process consistent with the invention. Such abias may facilitate heating of the joining areas by attracting thecharged ions in the plasma to the joining areas, which may encourageuniform coverage of the plasma over the joining areas. The bias appliedto the joining areas may be, for example, AC, DC, pulsed, continuous, orperiodic. The magnitude of the bias may be selected according to theparticular application. For example, the magnitude of the voltage mayrange from about 0.1 volts to about 100 volts, or even several hundredor thousands of volts, depending on the desired rate of attraction ofthe ionized species. Further, the bias may be positive, negative, oralternate therebetween. It will be appreciated that the parts or joiningareas may be placed on an electrically conductive plate and a potentialbias may be applied to the plate during a plasma-assisted processconsistent with the invention.

FIG. 12 shows a side cross-sectional view of illustrative plasma joiningcavity 460 formed in vessel 413 and parts 400 and 410 that are to bejoined to each other. Brazing ring 420 can include any type of brazingmaterial and may at least partially encircle part 410 and may abut anoutside surface of block 400. This may be the case, for example, whenpart 410 is a metallic tube and part 400 is a metallic block, which mayhave recess 415. In this case, end 412 of tube 410 may be inserted intorecess 415 in block 400. Alternatively, end 412 can just touch a portionof block 400 without any insertion. Brazing ring 420 can be formed, forexample, from metal, metal powder, flux, and any combination thereof. Asshown in FIG. 12, block 400, tube 410, and brazing ring 420 can beplaced in cavity 460, with tube 410 extending through opening 450 invessel 413. In this embodiment, joining can result from at least apartial melting and freezing of brazing ring 420. At least one plasmacatalyst (as described above) may also be placed within or near cavity460 to assist in the formation of the joining plasma, if desired.

FIG. 13 shows a cross-sectional view of plasma joining cavity 560 formedin vessel 513, as well as parts 500 and 510 that are to be joined toeach other. In this embodiment, non-joining areas 507 and 517, as wellas joining areas 505 and 515, when placed close to vessel 513, can fullyor partially seal aperture 520, although sealing is unnecessary. Joiningareas 505 and 515 can be spaced from inner surface 565 of cavity 560 bya distance of at least about λ/4, wherein λ is the wavelength of theapplied radiation used to form a plasma in the vicinity of joint areas505 and 515, to permit plasma formation at those areas. It will beappreciated that any plasma catalyst can be used to ignite, modulate,and sustain a plasma in cavity 560 consistent with this invention.

As described above, a temperature associated with the joining processmay be monitored by at least one temperature sensor and used to controlthe plasma-assisted joining process. The temperature of any part, suchas part 400 or part 410 of FIG. 12, or an inner surface of a cavity,such as surface 365 of FIG. 11, can therefore be monitored. If a colorpyrometer is used, the color of radiation emitted from a surface can bemonitored during the plasma-assisted joining process. When the brazingring melts, it may produce some additional emission in the plasma thatcan generate a sharp spike in the output of the temperature sensor. Whenthe temperature reaches a pre-determined level, for example, upondetecting a temperature spike, the spike can indicate that the brazingring has melted and the process is complete. In this case, controller 44can be programmed to turn off radiation source 26 immediately or after apreset delay time, if necessary, and close control valve 22. In anotherembodiment, control valve 22 may remain open for a period of time afterthe joining process for faster cooling of parts 11. Those skilled in theart will appreciate that the emission of additional radiation from theplasma upon melting of the brazing ring can be detected by any suitableoptical sensor (e.g., a photodiode, a photomultiplier tube, a hollowcathode lamp etc.), with or without a suitable wavelength selectivefilter.

In one embodiment consistent with this invention, a plasma can also beused to rapidly develop high-temperatures for the purpose of joiningmetals to non-metals, for example, metals to glass. Advantageously, thejoining of two different materials can involve localized heating andmelting near the joining areas only. Glass (or quartz) to metal seals,for example, can be used in making light bulbs (e.g., incandescent,quartz-halogen, sodium, mercury, and other types of light bulbs).

For glass-metal joints capable of maintaining hermetic seals over a widerange of temperatures, the two materials can have nearly the samethermal expansion coefficients. Alternatively, we can have “graded”seals in which the joining material (e.g., brazing ring material)composition varies gradually at the joining area such that there is noabrupt transition in the thermal expansion characteristics at any pointor region of the joint. In one embodiment, the joint can be formed whilemaintaining a gap between joining areas, with the powder metal/glassmixture placed in the gap with a suitable concentration variation. Afterthe mixture is placed in the gap, the joint can be fused withplasma-assisted heating consistent with this invention.

In another embodiment, the parts to be joined can be placed inindividual cavities formed in one or more radiation-transmissive blocks,such as ceramic blocks. At least one plasma catalyst can be placed ineach of the cavities to ignite and/or sustain the plasma there. In thisexample, radiation can be generated from one radiation source or severalcombined radiation sources. The radiation can be provided to theindividual cavities through an appropriately shaped waveguide (e.g., ahorn that connects the radiation source(s) to the individual cavitiesformed in a large ceramic block) or supplied directly without the use ofa waveguide.

In the foregoing described embodiments, various features are groupedtogether in a single embodiment for purposes of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description ofEmbodiments, with each claim standing on its own as a separate preferredembodiment of the invention.

1. A method of joining at least a first joining area of a first part anda second joining area of a second part, the method comprising: placingat least the first and second joining areas in proximity to one anothernear an aperture of a plasma-joining cavity, so that said first andsecond joining areas fully seal or partially seal said aperture; forminga plasma in the cavity by subjecting a gas to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of a plasmacatalyst; and sustaining the plasma at least until the first and secondjoining areas are joined by directing the radiation into the cavity. 2.The method as recited in claim 1, wherein placing at least the first andsecond joining areas near the aperture of and exterior to theplasma-joining cavity includes spacing each of said first and secondjoining areas a distance of at least λ/4 from an inner surface of saidplasma-joining cavity, where λ is a wavelength of the appliedelectromagnetic radiation.
 3. The method as recited in claim 1, furthercomprising making a time-averaged distribution of radiationsubstantially uniform within the cavity.
 4. The method as recited inclaim 1, further comprising monitoring an emission from the plasma forintensity, wherein sustaining the plasma is performed until a spikeappears in the emission intensity.
 5. The method as recited in claim 1,further comprising introducing a plasma catalyst into the plasma-joiningcavity through at least one ignition port.
 6. The method as recited inclaim 5, wherein a plasma catalyst component ratio in the plasmacatalyst can vary continuously or discontinuously throughout theplasma-joining cavity.
 7. The method as recited in claim 5, wherein theplasma catalyst includes multiple catalysts.
 8. The method as recited inclaim 5, wherein the plasma catalyst is suspended in a fluid beforebeing introduced into the plasma-joining cavity.
 9. The method asrecited in claim 5, wherein the plasma catalyst is uniformly orsubstantially uniformly distributed throughout the plasma-joining cavitywhen introduced into said plasma-joining cavity.
 10. The method asrecited in claim 9, wherein distribution of the plasma catalyst into theplasma-joining cavity is homogeneous, non-homogeneous, well-graded orgap-graded.
 11. The method as recited in claim 1, further comprisingcontouring an inner surface of the plasma-joining cavity or an innersurface of the aperture to the plasma-joining cavity to selectively forma plasma where elevated heating is desired.